fix travis build error with libcurl4-guntls-dev

inspired by https://github.com/jupyter/nbviewer/blob/master/.travis.yml
this repo is [passed](https://travis-ci.org/jupyter/nbviewer) in recent build, so i'm guessing `sudo apt-get update` will fix this issue but still unsure, please correct me :cherries: 

/cc @stephenmathieson